1. Field of the Invention
This invention relates to novel barbituric acid compounds and more particularly to such compounds which are useful as deep ultraviolet (DUV) photoresist sensitizers, to a general method of synthesis for these sensitizers, to positive photoresist compositions incorporating said sensitizers and to the method for providing positive photoresist layers which incorporate these sensitizers. The invention contemplates the provision of novel barbituric compounds, per se, although the invention will be described primarily with reference to the use of compounds of this kind with reference to positive photoresist compositions and to the application thereof in the manufacture of semiconductor components.
2. Description of the Prior Art
Photoresists are materials which change their solubility in a developer solution after the photoresist has been exposed to actinic radiation, such as to ultraviolet radiation. Photoresist compositions comprise a photosensitive compound (hereafter sometimes called sensitizer or photosensitizer), a film forming polymeric resin and a solvent. There are other types of compositions possible, such as a photosensitive polymer in an appropriate solvent for example. The photoresist composition is applied to a substrate which is to be patterned and the solvent is then usually removed, as with heat, leaving the photoresist as a thin film covering the substrate. As a consequence of the exposure to radiation of the photoresist, a different solubility rate results between the exposed and unexposed (masked over) portions of a resist film that yields a surface relief pattern after the development. Those photoresists which become more soluble in a developer solution in the exposed regions are referred to as "positive" photoresists. Those which become less soluble in the exposed regions are referred to as "negative" photoresists. The present invention deals with a class of those compounds suitable for use in positive photoresist compositions.
Positive photoresists typically comprise an aqueous alkali soluble resin, such as novolac resin or poly(phydroxystyrene), and a diazonaphthoquinone sulfonic acid ester sensitizer. The resin and sensitizer are applied such as by spin coating, spray coating, or other suitable means from an organic solvent or solvent mixture onto a substrate, such as a silicon wafer or a chrome-plated glass plate. The developer used to process the positive photoresists are aqueous alkaline solutions, such as sodium metasilicate, potassium hydroxide, tetramethyl ammonium hydroxide and ammonium hydroxide. The developer removes the areas of the coated photoresist film that have been exposed to light or other form of irradiation so as to produce a relief pattern in the photoresist film.
The application of a photosensitive film to various substrates is an essential step in the fabrication of integrated circuits. The substrates are generally silicon wafers which may have a thin oxide coating or other coating such as silicon nitride or aluminum. The photosensitive film is used to pattern the substrate in a series of steps including exposure (through a mask pattern), development to yield a relief pattern in the resist layer and substrate etch to transfer that pattern into the substrate material. It is essential that the mask pattern be accurately reproduced in the substrate etch pattern. To achieve this high degree of accuracy, the mask pattern must be well resolved by the photoresist layer. The laws of optics and diffraction dictate that resolution will improve as the wavelength of the irradiation is shortened. Thus, photoresists capable of operating in the deep ultraviolet (DUV) region (200-300 nm) will be capable of higher potential resolution than those resists limited to operating in the near ultraviolet (NUV) region (300-400 nm). Conventional photoresists employing novolac resins as the alkali soluble, film forming polymers are highly absorbing in the DUV region in films of approximately one micron thickness and so cannot be used in that region. Diazonaphthoquinone sulfonic acid esters are commonly used as sensitizers in conventional NUV photoresists. While these ester sensitizers are photoactive in the DUV, they exhibit several serious limitations for use as DUV photosensitizers. These sensitizers exhibit intense absorptions in the DUV region, making the resist composition excessively absorptive as well. These DUV absorptions are also poorly photobleached by the exposing radiation so that the film's absorbance is not greatly diminished during the irradiation process. Ideally, the sensitizer photoproduct should be nonabsorbing in the region of irradiation used to expose the resist so that all absorbed light does useful chemistry, thereby maximizing sensitivity. These prior art sensitizers also possess NUV absorption bands which allow them to be used in conventional NUV photoresists. However, this NUV response would be considered a drawback in a true DUV resist, as it would necessitate filtering the exposure source to remove long wavelength radiation to prevent degradation of the resolution.
Accordingly, it is apparent that a need exists for new photosensitive compounds and for formulations which contain sensitizers designed specifically to operate effectively in the DUV spectral region during the process of integrated circuit manufacture. The present invention discloses a class of compounds which is especially well suited to perform in the desired region.
Other attempts have been made to design a photoresist system for the DUV region. For example, Reich-manis, Wilkins, Chandross and Gooden, as described in UK Patent Application No. 2,099,168 have demonstrated several systems based on the photochemistry of ortho-nitrobenzyl groups attached to both polymers and sensitizer molecules. Another disclosure is that of the use of orthonitrobenzyl chemistry for photoresists as in U.S. Pat. No. 3,849,137.
Chemistry suitable for DUV photoresists also includes that of chain scission of high molecular weight polymer into lower molecular weight polymer. In this case, the energy of the DUV light is sufficient to rupture bonds in the polymer chain, resulting in lower molecular weight material of increased solubility. The most common example of this technique uses poly(methyl methacrylate). The primary drawbacks of this resist are the need for high exposure doses to yield a sufficient difference in molecular weights and the need to use an organic solvent as the developing medium.
Another example of DUV photoresist technology involves the use of DUV excimer lasers of high instantaneous fluence to ablate away selected areas of the photoresist film. This technique has been referred to as laser photo-ablation. See, for example, the U.S. Pat. No. 4,414,059.
Still another disclosure relating to DUV sensitizers is found in European Patent Application No. 0 129 694, which describes compounds based on diazohomotetramic acid for use in photoresist compositions.
Further background on photoresist processes and requirements, including reference to photoresists especially designed for the DUV region is presented in the American Chemical Society Symposium Series #266, Materials for Microlithography, L. F. Thompson, Ed., ACS 1984, M. J. Bowden, "A Perspective on Resist Materials for Fine-Line Lithography".
Also, while the 1,3 dimethyl-5-diazobarbituric acid has been disclosed by Von Bruno Kokel, et al. in the German publication Angew. Chem. 92(9) 754 (1980), neither the compounds of the invention nor the usefulness as sensitizers for the positive photoresist composition has been known prior to our discovery.